1. Field of the Invention
The present invention relates to a semiconductor device having a heat dissipation metal layer (PHS: Plated Heat Sink) provided on the back side thereof, and a method of producing the semiconductor device wherein devices are separated by laser cutting.
2. Description of the Related Art
FIG. 3A through FIG. 3k show a method of producing a semiconductor device having a heat dissipation metal layer of the prior art disclosed in PCT application No. JP/96/02758.
According to such a method, a first separation groove 3 is formed by etching the surface of a GaAs substrate 1, on which semiconductor elements have been formed in advance, by using a photoresist layer 2 as a mask (FIG. 3A), then a first metal layer is formed in the first separation groove 3 by plating or another process (FIG. 3B).
Then the GaAs substrate 1 is coated with a wax 5 and is bonded onto a carrier substrate 6 such as glass plate or sapphire plate, with the GaAs substrate 1 being polished on the back side to reduce the thickness thereof to about 20 to 30 .mu.m (FIG. 3C).
Next, a photoresist layer 14 is formed on the back side of the GaAs substrate 1 in a first patterning step so that an opening is provided on the back of the first separation groove 3 (FIG. 3D). The back side of the GaAs substrate 1 is etched by using the photoresist layer 14 as a mask until the bottom of a metal layer 4 in the first separation groove is exposed, thereby forming a second separation groove 33 (FIG. 3E).
Then, after removing the photoresist layer 14, a plated feeder layer 7 is formed over the entire back surface of the GaAs substrate 1 (FIG. 3F). A second metal layer 16 made of the same metal as the first metal layer is formed in the second separation groove 33 by plating, using a photoresist layer 15 formed in a second patterning step as a mask (FIG. 3G).
Then, a photoresist layer 17 having a width smaller than that of the second separation groove 33 is formed in the second separation groove 33, using a third patterning step, and a gold PHS layer 8 is formed on the back side by an electrolytic plating method, using the photoresist layer 17 as a mask (FIG. 3H). The GaAs substrate 1 is then separated from the carrier substrate 6 (FIG. 2I) and an expand film 10 is attached to the PHS layer 8 (FIG. 3J).
Finally, the first and the second metal layers in the first separation groove are severed in the first separation groove 3 side by laser cutting operation using YAG laser or the like, thereby separating the elements to obtain a semiconductor device forming the fused metal mass 12 at the periphery of the semiconductor device (FIG. 3K).
According to the method of producing the semiconductor device as described above where the semiconductor devices connected with each other by means of the first and the second metal layers are separated by laser cutting of the metal layers, since the semiconductor elements are connected with each other by the two metal layers, the connection are resistant to bending and cracking during the producing process, while deposition of foreign matters on the element surface and defective appearance of the elements can be reduced thereby improving the production yield of the semiconductor devices, compared to a method of separating devices by dicing.
In the conventional method, three patterning steps are required from the first through the third patterning steps after the polishing step in which the GaAs substrate 1 is turned into a thin film of about 20 to 30 .mu.m in thickness. In case the patterning steps are carried out a plurality of times, the wax 5 softens due to the heat generated during baking in the patterning processes and removal of photoresist, thus causing thermal stresses in the GaAs substrate 1 and mechanical stresses in the GaAs substrate 1 during contact exposure, thereby making cracks or other defects likely to occur in the GaAs substrate 1. Thus there has been a limitation to the improvement in the production yield and it has been difficult to reduce the production costs due to the complicated producing process.
While the use of the two metal layers has an advantage that bending and cracking of the metal layer can be reduced during the production steps, the thick film may cause poor appearance of the cut portion and the generation of devices having poor appearance has been impeding the improvement in the production yield.
There has also been such a problem that the first metal layer and the second metal layer peel off each other when the second metal layer is formed in an electrolytic plating process.